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Unformatted text preview: 1 GEL 1: Lecture 8: Volcanism & Volcanic Hazards ‘(Ch. 9 & several pages in Ch. 6 - read selectively and avoid all the jargon) Volcano architecture and eruption style Magma may crystallize at depth in the Earth (“intrusive” igneous rock) or extrude from a volcano as lava or pyroclastic debris (both “extrusive” igneous rock) Magma from a magma chamber reaches the surface either through a vertical chimney to the crater at the top, or through long cracks called fissures where the lava erupts from a ‘vent’ along the flanks of the volcano. - eruptions from the crater are called summit eruptions whereas eruptions along the flanks are called flank eruptions or fissure eruptions Composition of magma - source of the shape and eruption style of volcanoes All magmas are different. They each have a distinctive chemical makeup, based on their particular tectonic setting and their pathways upward toward the surface. The most important chemical component is the relative amount of silica (SiO 2 ). Magmas are distinguished on the basis of their silica (SiO 2 ) content. * The shape of a volcano and the composition of the lava or pyroclastic debris that is erupted is directly related to its silica content . Silica controls the viscosity of the magma within the chamber- viscosity is simply a measure of a fluid’s resistance to flow. Magma consists of 1) the liquid melt, 2) any crystals of minerals that might have formed from the melt as it cooled, and 3) abundant gases such as H 2 O, CO 2 , SO 2 , and H 2 S - up to 9% of a magma's composition may consist of gases - these gases are extremely important since they provide the ‘gas pressure’ that drives the explosive force of volcanic eruptions. (The more gases trapped in the magma, the more powerful the potential eruption.) - the higher the viscosity, the greater the concentration of trapped gases and thus the higher the potential explosivity of eruption 1) Low-silica magma- the lower the silica content, the more “runny” the lava (lower viscosity). The lower viscosity and thinner texture of the lava permits the continual release of gases from the magma, lowering the pressure and thus the potential for explosive eruption. - this type of low-silica lava builds broad, gently sloping shield volcanoes , typified by Hawaiian volcanoes . 2- the low viscosity contributes to fast-flowing “rivers” of lava that pour out of the central crater or vents along the flanks, and spread out laterally, maintaining the broad, low- relief profile of shield volcanoes. - this type of eruption is described as effusive- on the Big Island of Hawaii, near constant effusive eruptions of lava over the past several hundred thousand years have built an enormous volcanic edifice that rises from the seafloor - Mauna Kea & Mauna Loa are the biggest mountains on Earth, measuring ~32,000’ from their bases on the seafloor (> 13,700’ above sea level)....
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